As a conventional technology, a rotation detection device that detects the rotation of a gear by using a differential Hall sensor has been proposed (see Non-Patent Literature 1, for example).
The rotation detection device disclosed in Non-Patent Literature 1 includes a magnet that forms a magnetic field toward the tooth surface of a gear, and a sensor IC (ATS667LSG) disposed between the gear and the magnet and provided with a pair of magnetic detection elements for detecting a variation in the magnetic flux density accompanying the rotation of the gear in the radial direction of the gear, the sensor outputting a difference in the outputs from the pair of magnetic detection elements. With the rotation of the gear, a signal which is output from the sensor IC is periodically changed, whereby the rotating speed, angular speed, and the like of the gear can be detected.